A window blind for a sliding roof system may be attached underneath an opening in a vehicle roof to cover an opening in the vehicle roof to a greater or lesser extent according to the choice of the vehicle occupants. If the opening is to be exposed, the window blind is wound up on a winding shaft, which is biased by a spring in the wind-up direction of the shaft. The window blind can therefore be uncoiled against the biasing force of the spring to cover the opening. When the window blind lies underneath the opening in the vehicle roof, the window blind is prevented from sagging into the vehicle interior space by a guide rail on either side of the longitudinal edges of the window blind. Each guide rail receives a longitudinal edge of the window blind and holds the edge taut transverse to the shifting direction of the window blind.
One disadvantage of currently known window blinds is that the spring acting on the winding shaft must be sufficiently strong to be able to wind up the window blind reliably. At the same time, however, the biasing force of the spring must not displace the window blind after it is shifted to a desired position by the vehicle occupant. Due to the aging of the spring, the varying friction-related conditions that can occur during the lifespan of the window blind, and the various spring forces that act with the window blind being fully uncoiled from the winding shaft on the one hand and that act with the window blind being almost completely wound up on the winding shaft on the other hand, the numerous demands on the window blind system can be met only at large manufacturing expense.
There is a desire for a window blind that can be reliably wound up while still securely remaining in any given position set by the vehicle occupant without great expense. There is also a desire for a window blind that occupies a small installation space.